1. Field of Invention
The invention relates to processes for producing an underground zone 32 of fragmented and pervious material which may be used as a storage area, as a passage or aquifier, for treatment of materials, for ore body leaching or for recovery of carbonaceous materials from subterranean deposits. An example of the latter use is the extraction of oil and hydrocarbon gases from oil shale by in situ retorting, see Lekas U.S. Pat. No. 4,037,657.
2. Description of Prior Art
In one form of the invention disclosed in the above-referenced patent, a plurality of blast holes are drilled through the overburden and into the ore body, explosive charges placed in the holes and exploded to fragment the ore body. As a by-product of the fragmenting of the ore body to produce rubble, the surface of the ground above the rubble so formed expands upwardly with attendant cracking of the overburden. Accordingly, to produce an enclosed in situ retort, the cracks need be sealed, as by pumping a mud slurry or the like into the cracks or, alternatively, the top surface may be sealed by means of a layer of soil, plastic sheets or the like.
It has been found that, even where light charges are used to fragment the oil shale, near-surface retorts are incapable of enclosing gas under appreciable pressure--more than a few pounds per square inch. Even at low pressures of this magnitude, leakage is a serious problem if the overburden is significantly fractured, as happens if the surface is greatly domed over the retort zone. Sealing can be expensive and ecologically disadvantageous in that surface vegetation is destroyed and membranes remain after the retorting process is completed.
Another problem encountered in the prior art is the inefficient use of explosives to fragment the oil shale in the absence of a free face against which blasting can take place. If the explosives are to fragment the material by passage of waves which do not have a free face to operate against, the amount of explosives needed is greatly increased and the stress level likewise increased. Such stresses must necessarily impact the overburden and, moreover, the upper surface of the burden presents an acoustic mismatch, i.e., a free face. It is thus impracticable to avoid unwanted damage to the overburden.